The present invention relates to detent means for precisely registering one or more relative positions between a pair of bodies of which at least one has normally free movement. Such means are particularly useful for indexing the primary level position of the telescope of a level-transit surveying instrument.
The level-transit finds widespread use in the construction trade where it is generally employed to determine azimuth and elevation angles as well as to establish spirit level datum planes. It is in respect of this latter function of the instrument that there is advantageously employed some means of readily locking, or indexing, the telescope with respect to the alidade standard in order to reestablish, after making elevational sightings, the perpendicularity between the line of sight and the vertical spindle axis. The capability of retaining a high degree of precision after extended use is a basic requirement for indexing detent means in a datum function instrument such as the level-transit. Previously available detent assemblies have, among other disadvantages, lacked such long-term precision.
In the field of level-transit instruments there have been utilized various types of level index detents, yet none have performed with the reliable precision exhibited by the present invention. For example, there has been described in U.S. Pat. No. 2,607,994 detent means comprising a lever with plano-parallel sides retractably positionable between a pair of lands to establish an index of the level line of sight in such an instrument. The degree of ease with which the lever in such an arrangement may be set to index the instrument depends in great measure upon the clearance between the abutting lands; however, the greater that dimension is set to obtain ease of operation, the less precise becomes the indexing of the level position of the telescope. Wear at the relatively fixed and broad surfaced areas of contact between the lands and lever as well as between the lever and its pivot pin results, with continued use, in further loss of precision of indexing.
Another level-transit instrument described in U.S. Pat. No. 2,674,044 employs a spring-urged pawl in conjunction with a bevel-walled receiving slot in an attempt to achieve datum plane indexing with a greater degree of precision and convenience. The additional use of a beveled pawl pivot slot was suggested as a means of reducing loss of precision occasioned by wear at contacting surfaces. The extent of area and fixed location of such surfaces continued, however, to lead to loss of index precision.
A further difficulty experienced in previous instruments of the type noted resulted from the relatively inflexible locking actions of the detent means employed. Thus, once engaged by such means in level datum position the telescope of the instrument and its horizontal axis and bearings were susceptible to damage from any accidental blow or other excessive force tending to plunge the telescope. Although the bevels in the latter-noted instrument were suggested as a means of preventing such damage they were found to be of little efficacy for that purpose after a short period of use and resulting wear and seating of parts. The spring-triggered action of the pawl of that instrument further caused repeated detent- and bearing-stressing shock as a result instant locking each time the telescope was rapidly pivoted to the location of the index point.